Dogs show signs of joy after they figure out a problem -- akin to canine "Eureka!" moments -- says a new study by zoologists at the Swedish University of Agricultural Sciences. And it turns out puzzled pooches don't like being stumped: Dogs get frustrated, too.

The biologists trained twelve female beagles to press a lever, play a plastic piano key or push an object snout-first. Once a dog successfully used her device, she'd start to wag her tail more vigorously than usual. But when the researchers mixed up the items (the dogs knew only how to accomplish certain tasks) the dogs bit and gnawed on the strange devices -- a familiar feeling for anyone who's ever felt a bit disgruntled.

"From an evolutionary standpoint, it makes sense that animals should react emotionally to their own achievements," the authors write, because positive feelings can boost memory and learning. A dog's celebratory wag of the tail, then, might be a sign she's ready to face similar problems in the future.

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